1. Field of the Invention
The present invention relates to a developing apparatus for carrying out electrophotographic development used with a uni-component magnetic developer (or toner) and, more particularly, to a developing apparatus using, as toner carrier, a rotational sleeve accommodating a stationary magnet roller having a plurality of magnetic poles.
2. Related Art
A well-known developing apparatus uses a rotational sleeve accommodating a stationary magnet roller having a plurality of magnetic poles as toner carrier. In this developing apparatus, toner charged into and collected in a developer vessel is caused to be carried by utilizing magnetic carrying forces produced on the rotational sleeve, and the layer thickness is regulated by utilizing a toner layer thickness regulator called doctor blade, then the regulated toner layer being led to a developing position.
FIG. 3 shows the pole configuration structure of a stationary magnet roller 10, which is used as related art to the above well-known apparatus. The illustrated stationary magnet roller 10 has a four-pole structure constituted by:
a developing pole N1 facing a developing position; PA1 a shield pole S1 facing the inner surface of a wall 20a constituting a toner vessel bottom, which shields, via a predetermined gap, a portion of the outer periphery of a rotational sleeve 11 that is found downstream the developing pole N1 in the sleeve rotation direction; (as indicated by the arrow C) PA1 a blade-facing pole S2 facing a doctor blade for toner layer thickness restriction; and PA1 a carrier pole N2 located between the shield pole S1 and the blade-facing pole S2. PA1 a mixer 22 for stirring toner as uni-component magnetic developer; PA1 a rotational sleeve 11 accommodating a stationary magnet roller having a plurality of magnetic poles; and PA1 a toner layer thickness regulator 13 disposed upstream a developing position, at which image on a photo-sensitive drum A is developed, in the sleeve rotation direction; PA1 a developing pole N1 located near the developing position and facing the photo-sensitive drum A; PA1 a blade-facing pole S2 which is one of two separate poles of the opposite polarity to the developing pole N1 and facing the toner layer thickness regulator 13; and PA1 a shield pole S1 which is the other one of the two separate poles and located downstream the developing position in the sleeve rotation direction.
FIG. 4 shows a different example of stationary magnet roller 10, which has a five-pole structure having separate poles N2 and N3 located adjacent to each other and having the same polarity as and replacing the pole N2 shown in FIG. 3.
The above four-pole structure can find extensive applications to a commonly termed flying developing system using a uni-component magnetic toner. This structure, however, has such a drawback that it may sometimes fail to sufficiently stir and mix both return toner returning from the developing position, at which an image on a photo-sensitive drum A is developed, and fresh toner on the side of a toner vessel 20.
In the above five-pole structure which has the separate poles N2 and N3 located one adjacent to the other to utilize a repulsive magnetic field thereof, the inter-polar angle between the separate poles N2 and N3 is small. Therefore, although no problem arises where a two-component developer is used because of the fluidity of the toner is satisfactory in this case, in case of a developing apparatus used with a uni-component developer, the fluidity of toner is unsatisfactory, and also the bulk density of the developer is low, so that the separation performance is unsatisfactory.
The above drawbacks are increased the smaller the diameter of the stationary magnet roller 10, eventually making it impossible to effectively use the four- or five-pole structure.